This prospective study reports the incidence of hydatidiform mole (HM) in a population of 13,510 pregnancies in a Tokyo hospital over a 5-year period between 1989 and 1993. During this period all "products of conception" from first- and second-trimester abortions were histologically reviewed, and 76 hydropic placentas were retrieved and analyzed by flow cytometry (FCM). Of 23 specimens originally diagnosed as complete hydatidiform mole (CM), 21 were diploid, and two were aneuploid (nontriploid/tetraploid). Of 22 partial hydatidiform moles (PMs), 20 were triploid, and two were diploid. Of 31 hydropic abortions (HAs), 20 were diploid, nine were triploid, one was tetraploid, and one was aneuploid. As to the correlation between morphology and data of FCM, two PMs were reclassified as HA, and eight HAs as PM, giving a ratio of 1 CM to 1.22 PM (23:28 cases). The incidence of HM was 1:265 pregnancies, (CM, 1:587; PM, 1:483). Only one case (3.6%) of PM was suspected clinically. One specimen of persistent disease occurred following a diploid CM. In our retrospective histological and FCM study in which 172 cases diagnosed as HM were retrieved from surgical pathology files between 1981 and 1991, there were 129 CMs and 43 PMs (CMs:PMs = 3:1). These findings indicate that PM is a common but underdiagnosed condition. Almost all studies in the literature may have severely underreported the incidence of PM. It is suggested that during routine delivery and pathology examination only the most florid PMs are recognized, whereas most with subtler changes go undiagnosed.(ABSTRACT TRUNCATED AT 250 WORDS)